The invention relates to a method for controlling a corona ignition system of a cyclically operating internal combustion engine. Methods of this type are generally known in, e.g., US 2013/0319095 A1.
US 2011/0114071 A1 discloses a corona system for igniting a fuel/air mixture in a combustion chamber of an internal combustion engine by a corona discharge produced in the combustion chamber. This corona ignition system has an ignition electrode, which is surrounded by an insulator. The ignition electrode, the insulator and a sleeve surrounding the insulator together form an electrical capacitor. This capacitor is part of an electrical resonating circuit of the corona ignition device, which resonating circuit is excited with a high-frequency AC voltage for example from 30 kHz to 50 MHz. This results in a voltage excess at the ignition electrode so that a corona discharge forms at said electrode.
The high-frequency AC voltage is produced by a high-frequency generator. The input voltage of the high frequency generator is produced by a converter from the on-board supply voltage of the vehicle. The input voltage of the high-frequency generator generally is in the range from 100 V to 400 V in the case of known corona ignition systems.
A corona discharge creates ions and radicals in a fuel/air mixture in the combustion chamber of an engine. When a critical concentration of ions and radicals is reached, the fuel/air mixture ignites. The rate at which ions and radicals are created is dependent on the size of the corona discharge and the electrical power thereof. The size and power of a corona discharge can only increase up to a critical limit. If this limit is exceeded, the corona discharge transitions into an arc discharge or spark discharge.
Corona ignition systems are therefore controlled such that the corona discharge is as large as possible, and therefore the fuel/air mixture can be ignited as quickly as possible and the moment of ignition can thus be controlled as precisely as possible, however a breakdown of the corona discharge into an arc or spark discharge is avoided.
For this purpose, it is known from US 2011/0114071 A1 to measure the impedance of the resonating circuit. The impedance is compared with a fixed target value for the impedance, which is selected such that the corona ignition can be maintained without resulting in complete voltage breakdown.
This method has the disadvantage that the formation of the corona is not optimal and in particular an optimal size of the corona is not always achieved. The corona becomes larger, the closer the resonating circuit is operated to the breakdown voltage. In order to avoid the breakdown voltage in all circumstances, the target value of the impedance, which must not be exceeded, therefore must be so low that a voltage breakdown and therefore a spark discharge are avoided in any case.